U.S. patent application number 16/500731 was filed with the patent office on 2020-04-16 for acoustic sensor.
The applicant listed for this patent is TTP Plc. Invention is credited to Justin BUCKLAND, Harry BULLIVANT, Adam JACKSON.
Application Number | 20200116671 16/500731 |
Document ID | / |
Family ID | 58795665 |
Filed Date | 2020-04-16 |
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United States Patent
Application |
20200116671 |
Kind Code |
A1 |
BULLIVANT; Harry ; et
al. |
April 16, 2020 |
ACOUSTIC SENSOR
Abstract
An acoustic sensor, comprising: a side wall, closed by first and
second end walls to form a substantially cylindrical cavity for
containing a fluid, wherein a radius, a, of the cavity and an axial
height, h, of the cavity satisfies the inequality a/h is greater
than 1.2; a transmitter, operatively associated with one of the
first and second end walls; a receiver, operatively associated with
the other of the first and second end walls; and a first stiffener
plate, comprising an outer peripheral edge and an aperture which
defines an inner peripheral edge, and located on an outer face of
the first end wall such that the aperture overlies the axis of the
cavity; wherein: the first end wall comprises at least one
through-hole, located radially of the axis of the cavity between
the inner and outer peripheral edges of the first stiffener plate;
the first stiffener plate comprises at least one duct, which
connects the at least one through-hole, of the first end wall, to
at least one of the inner and outer peripheral edges of the first
stiffener plate, thereby to provide at least one fluid passageway
between the cavity and the external surroundings of the acoustic
sensor via the first end wall; and in use: the transmitter causes
oscillatory motion, of the one of the first and second end walls
with which the transmitter is associated, in a direction
substantially perpendicular to the plane of that end wall, such
that axial oscillations of that end wall drive substantially radial
oscillations of a fluid pressure in the cavity; and the
substantially radial oscillations in the pressure of the fluid
drive oscillatory motion of the other of the first and second end
walls, with which the receiver is associated, generating an
electrical signal.
Inventors: |
BULLIVANT; Harry; (Royston,
Hertfordshire, GB) ; JACKSON; Adam; (Royston,
Hertfordshire, GB) ; BUCKLAND; Justin; (Royston,
Hertfordshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TTP Plc |
Royston, Hertfordshire |
|
GB |
|
|
Family ID: |
58795665 |
Appl. No.: |
16/500731 |
Filed: |
April 11, 2018 |
PCT Filed: |
April 11, 2018 |
PCT NO: |
PCT/GB2018/050963 |
371 Date: |
October 3, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2291/0212 20130101;
G01N 29/28 20130101; G01N 2291/014 20130101; G01N 29/2437 20130101;
G01N 29/222 20130101; G01N 29/036 20130101; G01N 29/024
20130101 |
International
Class: |
G01N 29/22 20060101
G01N029/22; G01N 29/036 20060101 G01N029/036; G01N 29/24 20060101
G01N029/24; G01N 29/28 20060101 G01N029/28 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 2017 |
GB |
1706393.4 |
Claims
1. An acoustic sensor, comprising: a side wall, closed by first and
second end walls to form a substantially cylindrical cavity for
containing a fluid, wherein a radius, a, of the cavity and an axial
height, h, of the cavity satisfies the inequality a/h is greater
than 1.2; a transmitter, operatively associated with one of the
first and second end walls; a receiver, operatively associated with
the other of the first and second end walls; and a first stiffener
plate, comprising an outer peripheral edge and an aperture which
defines an inner peripheral edge, and located on an outer face of
the first end wall such that the aperture overlies the axis of the
cavity; wherein: the first end wall comprises at least one
through-hole, located radially of the axis of the cavity between
the inner and outer peripheral edges of the first stiffener plate;
the first stiffener plate comprises at least one duct, which
connects the at least one through-hole, of the first end wall, to
at least one of the inner and outer peripheral edges of the first
stiffener plate, thereby to provide at least one fluid passageway
between the cavity and the external surroundings of the acoustic
sensor via the first end wall; and in use: the transmitter causes
oscillatory motion, of the one of the first and second end walls
with which the transmitter is associated, in a direction
substantially perpendicular to the plane of that end wall, such
that axial oscillations of that end wall drive substantially radial
oscillations of a fluid pressure in the cavity; and the
substantially radial oscillations in the pressure of the fluid
drive oscillatory motion of the other of the first and second end
walls, with which the receiver is associated, generating an
electrical signal.
2. An acoustic sensor according to claim 1, wherein the at least
one duct, of the first stiffener plate, connects the at least one
through-hole, of the first end wall, to the inner peripheral edge
of the first stiffener plate.
3. An acoustic sensor according to claim 1, wherein the at least
one duct, of the first stiffener plate, connects the at least one
through-hole, of the first end wall, to the outer peripheral edge
of the first stiffener plate.
4. An acoustic sensor according to claim 1, wherein at least a
portion of the length of the at least one duct, of the first
stiffener plate, extends normal to the axis of the cavity so as to
be parallel with the plane of the first end wall.
5. An acoustic sensor according to claim 4, wherein the entire
length of the at least one duct, of the first stiffener plate,
extends normal to the axis of the cavity so as to be parallel with
the plane of the first end wall.
6. An acoustic sensor according to claim 5, wherein the at least
one duct, of the first stiffener plate, extends along the said
outer face of the first end wall, so as to take the shortest route
between the at least one through-hole, of the first end wall, and
the respective inner or outer peripheral edge of the first
stiffener plate.
7. An acoustic sensor according to claim 1, wherein: the first end
wall comprises a plurality of the said through-holes; and the first
stiffener plate comprises a plurality of the said ducts, each duct,
of the first stiffener plate, connecting a respective one of the
plurality of through-holes, of the first end wall, to the
respective at least one of the inner and outer peripheral edges of
the first stiffener plate, thereby to provide a plurality of fluid
passageways between the cavity and the external surroundings of the
acoustic sensor via the first end wall.
8. An acoustic sensor according to claim 7, wherein one or more of
the said through-holes, of the first end wall, is located at a
nodal pressure position of the radial pressure oscillation which is
generated in the cavity in use.
9. An acoustic sensor according to claim 8, wherein the said nodal
pressure position is a radially innermost nodal pressure position,
of the radial pressure oscillation which is generated in the cavity
in use.
10. An acoustic sensor according to claim 7, wherein a plurality of
the said through-holes, of the first end wall, are arranged so as
to be equally spaced around a circumference of a circle which is
axially concentric with the cavity.
11. An acoustic sensor according to claim 1, wherein an axial
thickness of the first stiffener plate is greater than an axial
thickness of the first end wall.
12. An acoustic sensor according to claim 1, wherein the
transmitter is operatively associated with the first end wall by
being attached to the outer face of the first end wall at a
location which is within the inner peripheral edge of the first
stiffener plate.
13. An acoustic sensor according to claim 1, wherein the inner
peripheral edge of the first stiffener plate defines a circle.
14. An acoustic sensor according to claim 13, wherein the outer
peripheral edge of the first stiffener plate defines a circle.
15. An acoustic sensor according to claim 14, wherein the outer
peripheral edge of the first stiffener plate defines a
rectangle.
16. An acoustic sensor according to claim 15, wherein the aperture
of the first stiffener plate overlies the axis of the cavity such
that the aperture is axially concentric with the cavity.
17. An acoustic sensor according to claim 16, wherein the first
stiffener plate and the first end wall are of unitary
construction.
18. An acoustic sensor according to claim 17, wherein the first
stiffener plate is attached to the outer face of the first end
wall.
19. An acoustic sensor according to claim 1, comprising: a second
stiffener plate, comprising an outer peripheral edge and an
aperture which defines an inner peripheral edge, and located on an
outer face of the second end wall such that the aperture overlies
the axis of the cavity.
20. An acoustic sensor according to claim 19, wherein: the second
end wall comprises at least one through-hole, located radially of
the axis of the cavity between the inner and outer peripheral edges
of the second stiffener plate; and the second stiffener plate
comprises at least one duct, which connects the at least one
through-hole, of the second end wall, to at least one of the inner
and outer peripheral edges of the second stiffener plate, thereby
to provide at least one fluid passageway between the cavity and the
external surroundings of the acoustic sensor via the second end
wall.
21. An acoustic sensor according to claim 20, wherein the at least
one duct, of the second stiffener plate, connects the at least one
through-hole, of the second end wall, to the inner peripheral edge
of the second stiffener plate.
22. An acoustic sensor according to claim 20, wherein the at least
one duct, of the second stiffener plate, connects the at least one
through-hole, of the second end wall, to the outer peripheral edge
of the second stiffener plate.
23. An acoustic sensor according to claim 20, wherein at least a
portion of the length of the at least one duct, of the second
stiffener plate, extends normal to the axis of the cavity so as to
be parallel with the plane of the second end wall.
24. An acoustic sensor according to claim 23, wherein the entire
length of the at least one duct, of the second stiffener plate,
extends normal to the axis of the cavity so as to be parallel with
the plane of the second end wall.
25. An acoustic sensor according to claim 24, wherein the at least
one duct, of the second stiffener plate, extends along the said
outer face of the second end wall, so as to take the shortest route
between the at least one through-hole, of the second end wall, and
the respective inner or outer peripheral edge of the second
stiffener plate.
26. An acoustic sensor according to claim 20, wherein: the second
end wall comprises a plurality of the said through-holes; and the
second stiffener plate comprises a plurality of the said ducts,
each duct, of the second stiffener plate, connecting a respective
one of the plurality of through-holes, of the second end wall, to
the respective at least one of the inner and outer peripheral edges
of the second stiffener plate, thereby to provide a plurality of
fluid passageways between the cavity and the external surroundings
of the acoustic sensor via the second end wall.
27. An acoustic sensor according to claim 26, wherein one or more
of the said through-holes, of the second end wall, is located at a
nodal pressure position of the radial pressure oscillation which is
generated in the cavity in use.
28. An acoustic sensor according to claim 27, wherein the said
nodal pressure position is a radially innermost nodal pressure
position, of the radial pressure oscillation which is generated in
the cavity in use.
29. An acoustic sensor according to claim 26, wherein a plurality
of the said through-holes, of the second end wall, are arranged so
as to be equally spaced around a circumference of a circle which is
axially concentric with the cavity.
30. An acoustic sensor according to claim 19, wherein an axial
thickness of the second stiffener plate is greater than an axial
thickness of the second end wall.
31. An acoustic sensor according to claim 19, wherein the receiver
is operatively associated with the second end wall by being
attached to the said outer face of the second end wall at a
location which is within the inner peripheral edge of the second
stiffener plate.
32. An acoustic sensor according to claim 19, wherein the inner
peripheral edge of the second stiffener plate defines a circle.
33. An acoustic sensor according to claim 19, wherein the outer
peripheral edge of the second stiffener plate defines a circle.
34. An acoustic sensor according to claim 19, wherein the outer
peripheral edge of the second stiffener plate defines a
rectangle.
35. An acoustic sensor according to claim 19, wherein the aperture
of the second stiffener plate overlies the axis of the cavity such
that the aperture is axially concentric with the cavity.
36. An acoustic sensor according to claim 19, wherein the second
stiffener plate and the second end wall are of unitary
construction.
37. An acoustic sensor according to claim 19, wherein the second
stiffener plate is attached to the outer face of the second end
wall.
Description
RELATED APPLICATIONS
[0001] The present application is a national stage application
under 35 U.S.C. .sctn. 371 of International Application No.
PCT/GB2018/050963, filed 11 Apr. 2018, which claims priority to
Great Britain Patent Application No. 1706393.4, filed 21 Apr. 2017.
The above referenced applications are hereby incorporated by
reference into the present application in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to an acoustic sensor, and in
particular to an acoustic sensor having a generally disc-shaped
acoustic cavity and a duct arrangement to allow fluid to enter and
leave the cavity.
BACKGROUND OF THE INVENTION
[0003] Devices which determine the composition of a gas mixture by
measuring the speed of sound in that mixture are well known in the
prior art. The two most prevalent techniques are the time-of-flight
technique and the resonant cavity technique. The drawbacks and
limitations of each type of device are summarised below.
[0004] A time-of-flight device uses a pair of transducers to
transmit and receive short (typically on the order of microseconds)
pulses of acoustic energy. Alternatively a single transducer can be
used to send and receive in a pulse-echo mode. The speed of sound
is determined by measuring the time taken for these pulses to
travel a known distance through the test fluid. In the devices
referenced above, the composition of a binary gas mixture is
determined from this measurement. Typical limitations of such
devices include signal attenuation, echoes, dimensional stability,
poor temperature compensation, parasitic conduction, and poor pulse
shaping and pulse shape distortion. These problems limit both the
performance and long-term stability of time-of-flight devices.
[0005] Resonant cavity devices measure the resonant frequency of an
acoustic cavity. When the acoustic cavity is filled with a fluid,
the resonant frequency of the cavity directly relates to the
composition and physical properties of the fluid. A limitation of
the devices described in the prior art is the difficulty of
efficiently exciting a single resonant mode in the cavity. Many
competing resonant modes may exist within the sensor including
radial, axial and angular modes. Excitation of multiple modes
complicates the interpretation of the output of the sensor.
[0006] Poor coupling between the, typically axial, motion of the
transmitting transducer into the resonant mode of the cavity, and
poor coupling between the resonant mode of the cavity and the
receiving transducer produces a weak signal. Sources of electrical
noise in the receiver such as parasitic signals passing from the
transmitting transducer to the receiver through the structure of
the device, or mechanical resonance of components such as the
diaphragm, can be of a magnitude comparable to the signal
itself.
[0007] The shortcomings of the above resonant sensors are shared by
devices based upon a Helmholtz oscillator. In such a device the air
in the neck of the aperture of a cavity vibrates causing pressure
oscillations in that cavity which oppose the motion of the air in
the neck, leading to simple harmonic motion. The change in acoustic
intensity resultant from changing density and speed of sound of the
fluid in the cavity may be used to measure the composition of a
mixture of hydrogen and air. This has the additional disadvantage
that the single aperture in the resonant cavity prevents a flow of
fluid from passing through the sensor cavity, which is desirable
for a fast response time and ease of integration into a fluidic
system.
[0008] Given the shortcomings of time-of-flight and existing
resonant cavity speed-of-sound sensors, there is a need for
speed-of-sound sensor capable of efficiently generating a resonant
oscillation significantly larger than competing resonant modes and
parasitic oscillations. The efficient generation of a large
amplitude radial mode pressure oscillation at the resonant
frequency of the cavity overcomes many of the limitations of the
prior art.
[0009] The efficient generation of resonant acoustic standing waves
has been addressed in the field of fluid pumping. Pumps are known
which have substantially disc shaped cavities with high aspect
ratios (i.e. the ratio of the radius of the cavity to the height of
the cavity) and which in operation generate a resonant acoustic
standing wave in those cavities.
[0010] A pump disclosed in FIG. 1 of WO2006/111775 has a
substantially cylindrical cavity 11 comprising a side wall 14
closed at each end by end walls 12, 13. The pump also comprises an
actuator 20 that drives one or both of the end walls to oscillate
in a direction substantially perpendicular to the surface of the
plane of the end walls, referred to hereinafter as "axial
oscillations". In this geometry, the mechanical stiffness of the
actuator is well matched to the acoustic impedance of the cavity,
enabling efficient generation of a high amplitude pressure
oscillation.
[0011] The efficient generation of a pressure oscillation in such a
cavity is further dependent on the matching of the spatial profile
of the fluid oscillation in the cavity and the motion of the driven
end wall. When the spatial profiles are well matched, work done by
the actuator on the fluid in the cavity adds constructively,
thereby enhancing the amplitude of the pressure oscillation in the
cavity and delivering improved pump efficiency, referred to herein
as mode-shape matching. Conversely, in a pump where the spatial
profiles are poorly matched, work done by some regions of the end
wall on the fluid reduces rather than enhances the amplitude of the
fluid pressure oscillation in the fluid within the cavity. Thus,
the useful work done by the actuator on the fluid is reduced and
the pump becomes less efficient.
[0012] The acoustic gas sensor outlined in WO2013/083978 has
demonstrated feasibility of a speed-of-sound fluid sensor that
overcomes many of the limitations of prior art by utilising radial
pressure oscillations in a substantially cylindrical cavity formed
by a side wall and two end walls. A transducer, referred to
hereinafter as the "transmitter", is operatively associated with
one of the end walls to cause an oscillatory motion of the
associated end wall whereby axial oscillations of the end wall
drive radial oscillations of the fluid pressure in the cavity. The
radial pressure oscillation in the cavity generates axial motion of
the second end wall which is operatively associated with a second
transducer, referred to hereinafter as the "receiver", which in
turn generates an oscillatory electrical signal. By comparing the
signal used to drive the transmitter to the signal generated by the
receiver, properties of the fluid in the acoustic cavity can be
determined. In operation both the mechanical stiffness of the
transmitter and the receiver of such a device are deliberately
matched to the acoustic impedance of the disc shaped volume of
fluid in the cavity. This disc shaped geometry is also suitable for
achieving good spatial matching between the displacement profiles
of the transmitter and receiver and the radial fluid pressure
oscillation in the cavity. The combination of these properties
enables efficient generation of high amplitude pressure
oscillations by the transmitter and efficient generation of an
output signal from the receiver, overcoming many of the limitations
of the prior art.
[0013] In sensing applications, it is desirable to monitor a broad
range of gasses with a wide range of speeds of sound, including
those with high speeds of sound such as helium and hydrogen, and
those with low speeds of sound such as carbon dioxide. These gases
will have acoustic resonances covering a wide range of frequencies.
In order for a resonant acoustic gas sensor to work reliably and
accurately, the mechanical resonant frequencies of the sensor must
not be in the frequency range over which the acoustic resonant
frequency associated with a particular gas might be expected. This
is because mechanical resonant modes that can be excited by motion
of the transmitting transducer cause parasitic signals to pass from
the transmitting transducer to the receiver through the structure
of the device. These parasitic signals can be of a magnitude at
least comparable to the signal associated with acoustic resonance
that is transmitted through the fluid in the cavity itself. In
practice, the widest frequency range in which there are no
parasitic structural resonances is that between zero and the lowest
mechanical resonant frequency. As such it is desirable for the
acoustic resonance to be located in this range to enable as wide an
operational frequency range as possible.
[0014] For the resonant acoustic sensor described in WO2013/083978
based on a radial pressure mode described above, the acoustic
resonant frequency is linearly proportional to gas speed of sound
for a given cavity diameter. This means that for gases with a high
speed of sound, the mechanical resonant frequencies of the sensor
must be increased. Increasing the cavity diameter to lower the
acoustic resonance may be considered but is impractical in many
instances because the size of the sensor cannot be increased.
Furthermore, in general, increasing the cavity diameter lowers the
mechanical resonant frequencies of the sensor proportionally more
than the acoustic resonance, thereby narrowing the operational
range of speeds of sound of the sensor.
[0015] The resonant frequencies of the mechanical modes of the
sensor can be increased by stiffening the structure. However,
crucially, many approaches to stiffening the structure result in
poorer sensor performance overall, as other desirable properties of
the sensor are worsened as a result. This means that these
approaches lead to a compromise between sensor attributes, as
detailed below.
[0016] The sensor comprises a cylindrical cavity bounded by a side
wall and two end walls containing inlet holes and with
piezoelectric discs attached. The inlet holes allow fluid to enter
the cavity and are preferably positioned at nodal positions of the
operational acoustic mode. This is to ensure that acoustic damping
introduced by viscous losses arising from oscillatory motion of the
fluid in and out of these holes is minimised. Acoustic damping
lowers and broadens the resonant peak, which worsens the sensor's
accuracy and signal-to-noise ratio. Preferably the length of these
inlet holes is small, to ensure that fluid diffuses into the cavity
quickly. The response time of the sensor is governed by how quickly
fluid in the cavity can be replaced and it is desirable for the
response time of the sensor to be as short as possible.
[0017] The end walls are typically supported by rigid structures,
with dimensions chosen to control the structural resonant
frequencies of the sensor and to improve mode-shape matching. For
example, in the case where the first order radial acoustic
resonance is driven and detected, the end wall is clamped along a
circular perimeter, herein `transducer diameter`, inboard of the
diameter of the cavity itself but axially concentric with it. This
constrains motion of the end wall in regions between the cavity
diameter and transducer diameter, but permits motion of the end
wall in regions within the transducer diameter. This clamping is
typically achieved by an annular member attached to the end
wall.
[0018] The first structural resonant frequency of the sensor can be
increased by reducing the area over which there is motion of the
end walls. That is, the structure can be stiffened by reducing the
diameter of the transducer. In practice this is achieved by
substantially thickening the end walls over regions where end wall
motion is to be limited. However, in order to maximise the signal
to noise ratio of the sensor, the inlet holes must be positioned at
a nodal pressure position of the operational acoustic mode.
Therefore, although reducing the diameter of the transmit/receive
transducers stiffens the structure, the pitch circle diameter of
the inlet holes may need to be reduced to accommodate this stiffer
structure and avoid the inlet holes being blocked by material
required to stiffen the structure. Moving these inlet holes from
nodal positions increases acoustic damping and worsens the signal
to noise ratio of the sensor.
[0019] The structure can also be stiffened by increasing the
thickness of the transducers, without having to reduce their
diameter. That is, the entirety of the end wall can be thickened,
not just in regions where end wall motion is to be limited.
However, increasing the thickness of the transducer has a
proportionally smaller impact on increasing the structural
stiffness than reducing the diameter of the transducer. The
resonant frequency of the transducer is proportional to its
thickness divided by the square of its diameter. Therefore to
achieve the same increase in resonant frequency, the fractional
increase in the transducer thickness must be twice the fractional
decrease in the transducer diameter. A thicker end wall reduces the
displacement of the transducer for a given input energy and
consequently reduces the mechanical-acoustic coupling. This results
in a weaker acoustic resonance signal and reduced signal-to-noise
ratio for the same input power.
[0020] Inlet holes are preferably located near the pressure node of
the first radial acoustic resonance of the cavity, that is on a
circle with diameter approximately 0.63 times the cavity diameter.
If the positions of the inlet holes are fixed on this location, and
if the diameter of the transducer has to be reduced in order to
increase the resonant frequencies of the mechanical modes of the
sensor, then one solution is to create a set of substantially
vertical holes through the thicker regions of the end wall. This
allows gas to access the inlet holes without needing to change
their position. However, in doing so the response time of the
sensor increases significantly because these holes act as long,
narrow tubes through which gas must diffuse. Widening the diameter
of these holes to improve the response time is undesirable because
it reduces the stiffness of the structure and reduces its
mechanical resonance frequencies, negating the effect of a smaller
transducer diameter. The same is true of reducing the height of the
rigid structure, which also reduces mechanical resonance
frequencies.
[0021] A separate solution is to add inlet holes at the
circumference of the cavity, leaving the transducers unmodified.
However, as these holes are not at nodal positions, they must be
small to avoid adding significant acoustic damping. Small inlet
holes result in slow diffusion of gas into the cavity, so this
introduces a compromise between response time and signal to noise
ratio.
[0022] As such, for radial acoustic resonance-based sensors, there
remains a need to monitor high speed of sound gases without
compromising on the response time or signal-to-noise ratio of the
sensor.
SUMMARY OF THE INVENTION
[0023] According to an aspect of the invention, there is provided
an acoustic sensor, comprising: a side wall, closed by first and
second end walls to form a substantially cylindrical cavity for
containing a fluid, wherein a radius, a, of the cavity and an axial
height, h, of the cavity satisfies the inequality a/h is greater
than 1.2; a transmitter, operatively associated with one of the
first and second end walls; a receiver, operatively associated with
the other of the first and second end walls; and a first stiffener
plate, comprising an outer peripheral edge and an aperture which
defines an inner peripheral edge, and located on an outer face of
the first end wall such that the aperture overlies the axis of the
cavity; wherein: the first end wall comprises at least one
through-hole, located radially of the axis of the cavity between
the inner and outer peripheral edges of the first stiffener plate;
the first stiffener plate comprises at least one duct, which
connects the at least one through-hole, of the first end wall, to
at least one of the inner and outer peripheral edges of the first
stiffener plate, thereby to provide at least one fluid passageway
between the cavity and the external surroundings of the acoustic
sensor via the first end wall; and in use: the transmitter causes
oscillatory motion, of the one of the first and second end walls
with which the transmitter is associated, in a direction
substantially perpendicular to the plane of that end wall, such
that axial oscillations of that end wall drive substantially radial
oscillations of a fluid pressure in the cavity; and the
substantially radial oscillations in the pressure of the fluid
drive oscillatory motion of the other of the first and second end
walls, with which the receiver is associated, generating an
electrical signal.
[0024] The provision of a ducted stiffener plate advantageously
allows that the structural stiffness of the sensor may be enhanced,
while the through-hole(s) of the cavity end wall may be provided in
the optimal location, for the best performance of the sensor using
a broad range of fluids.
[0025] The at least one duct, of the first stiffener plate, may
connect the at least one through-hole, of the first end wall, to
the inner peripheral edge of the first stiffener plate. The at
least one duct, of the first stiffener plate, may connect the at
least one through-hole, of the first end wall, to the outer
peripheral edge of the first stiffener plate.
[0026] At least a portion of the length of the at least one duct,
of the first stiffener plate, may extend normal to the axis of the
cavity so as to be parallel with the plane of the first end wall.
The entire length of the at least one duct, of the first stiffener
plate, may extend normal to the axis of the cavity so as to be
parallel with the plane of the first end wall. The at least one
duct, of the first stiffener plate, may extend along the said outer
face of the first end wall, so as to take the shortest route
between the at least one through-hole, of the first end wall, and
the respective inner or outer peripheral edge of the first
stiffener plate.
[0027] The first end wall may comprise a plurality of the said
through-holes; and the first stiffener plate may comprise a
plurality of the said ducts, each duct, of the first stiffener
plate, connecting a respective one of the plurality of
through-holes, of the first end wall, to the respective at least
one of the inner and outer peripheral edges of the first stiffener
plate, thereby to provide a plurality of fluid passageways between
the cavity and the external surroundings of the acoustic sensor via
the first end wall.
[0028] One or more of the said through-holes, of the first end
wall, may be located at a nodal pressure position of the radial
pressure oscillation which is generated in the cavity in use. The
said nodal pressure position may be a radially innermost nodal
pressure position, of the radial pressure oscillation which is
generated in the cavity in use.
[0029] A plurality of the said through-holes, of the first end
wall, may be arranged so as to be equally spaced around a
circumference of a circle which is axially concentric with the
cavity.
[0030] An axial thickness of the first stiffener plate may be
greater than an axial thickness of the first end wall.
[0031] The transmitter may be operatively associated with the first
end wall by being attached to the said outer face of the first end
wall at a location which is within the inner peripheral edge of the
first stiffener plate.
[0032] The inner peripheral edge of the first stiffener plate may
define a circle. The outer peripheral edge of the first stiffener
plate may define a circle. The outer peripheral edge of the first
stiffener plate may define a rectangle. In the case that each of
the inner and outer peripheral edges of the first stiffener plate
defines a circle, the first stiffener plate may be described as a
first stiffener ring.
[0033] An axis of the aperture of the first stiffener plate may be
parallel with the axis of the cavity. The aperture of the first
stiffener plate may overlie the axis of the cavity such that the
aperture is axially concentric with the cavity.
[0034] The first stiffener plate and the first end wall may be of
unitary construction. The first stiffener plate may be attached to
the outer face of the first end wall.
[0035] The acoustic sensor may comprise: a second stiffener plate,
comprising an outer peripheral edge and an aperture which defines
an inner peripheral edge, and located on an outer face of the
second end wall such that the aperture overlies the axis of the
cavity.
[0036] The second end wall may comprise at least one through-hole,
located radially of the axis of the cavity between the inner and
outer peripheral edges of the second stiffener plate; and the
second stiffener plate may comprise at least one duct, which
connects the at least one through-hole, of the second end wall, to
at least one of the inner and outer peripheral edges of the second
stiffener plate, thereby to provide at least one fluid passageway
between the cavity and the external surroundings of the acoustic
sensor via the second end wall.
[0037] The at least one duct, of the second stiffener plate, may
connect the at least one through-hole, of the second end wall, to
the inner peripheral edge of the second stiffener plate. The at
least one duct, of the second stiffener plate, may connect the at
least one through-hole, of the second end wall, to the outer
peripheral edge of the second stiffener plate.
[0038] At least a portion of the length of the at least one duct,
of the second stiffener plate, may extend normal to the axis of the
cavity so as to be parallel with the plane of the second end wall.
The entire length of the at least one duct, of the second stiffener
plate, may extend normal to the axis of the cavity so as to be
parallel with the plane of the second end wall. The at least one
duct, of the second stiffener plate, may extend along the said
outer face of the second end wall, so as to take the shortest route
between the at least one through-hole, of the second end wall, and
the respective inner or outer peripheral edge of the second
stiffener plate.
[0039] The second end wall may comprise a plurality of the said
through-holes; and the second stiffener plate may comprise a
plurality of the said ducts, each duct, of the second stiffener
plate, connecting a respective one of the plurality of
through-holes, of the second end wall, to the respective at least
one of the inner and outer peripheral edges of the second stiffener
plate, thereby to provide a plurality of fluid passageways between
the cavity and the external surroundings of the acoustic sensor via
the second end wall.
[0040] One or more of the said through-holes, of the second end
wall, may be located at a nodal pressure position of the radial
pressure oscillation which is generated in the cavity in use. The
said nodal pressure position may be a radially innermost nodal
pressure position, of the radial pressure oscillation which is
generated in the cavity in use.
[0041] A plurality of the said through-holes, of the second end
wall, may be arranged so as to be equally spaced around a
circumference of a circle which is axially concentric with the
cavity.
[0042] An axial thickness of the second stiffener plate may be
greater than an axial thickness of the second end wall.
[0043] The receiver may be operatively associated with the second
end wall by being attached to the said outer face of the second end
wall at a location which is within the inner peripheral edge of the
second stiffener plate.
[0044] The inner peripheral edge of the second stiffener plate may
define a circle. The outer peripheral edge of the second stiffener
plate may define a circle. The outer peripheral edge of the second
stiffener plate may define a rectangle. In the case that each of
the inner and outer peripheral edges of the second stiffener plate
defines a circle, the second stiffener plate may be described as a
second stiffener ring.
[0045] An axis of the aperture of the second stiffener plate may be
parallel with the axis of the cavity. The aperture of the second
stiffener plate may overlie the axis of the cavity such that the
aperture is axially concentric with the cavity.
[0046] The second stiffener plate and the second end wall may be of
unitary construction. The second stiffener plate may be attached to
the outer face of the second end wall.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] Embodiments will now be described, by way of example, with
reference to the accompanying figures in which:
[0048] FIG. 1A shows a schematic cross-section of an embodiment of
an acoustic sensor according to the invention;
[0049] FIGS. 1B to 1D show respectively the displacement profile of
an end wall of the sensor, a graph of the pressure oscillations
within the cavity of the sensor, and the displacement profile of
another end wall of the sensor;
[0050] FIGS. 2A and 2B show respectively typical phase difference
and gain between transmitter and receiver in the vicinity of the
cavity resonance for an embodiment of the acoustic sensor and
typical gain between transmitter and receiver over a frequency
range which includes the cavity resonance and lowest mechanical
mode of the sensor;
[0051] FIGS. 3A and 3B show respective bottom views of two examples
of a backer, or stiffener plate, of the sensor;
[0052] FIG. 3C shows a cross-section of an embodiment of the
acoustic sensor wherein a fluidic path length between the cavity
and sensor surroundings is minimised;
[0053] FIG. 4 shows a bottom view of an exemplary end wall; and
[0054] FIG. 5 shows an embodiment of the acoustic sensor where one
end wall and associated backer, or stiffener plate, geometry is
different to another end wall and associated backer, or stiffener
plate, geometry.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0055] In the following detailed description of several
illustrative embodiments, reference is made to the accompanying
drawings that form a part hereof, and in which is shown by way of
illustration specific preferred embodiments in which the invention
may be practised. These embodiments are described in sufficient
detail to enable those skilled in the art to practise the
invention, and it is understood that other embodiments may be
utilised and that logical structural, mechanical, electrical and
chemical changes may be made without departing from the scope of
the invention. To avoid detail not necessary to enable those
skilled in the art to practice the embodiments described herein,
the description may omit certain information known to those skilled
in the art. The following detailed description is, therefore, not
to be taken in a limiting sense.
[0056] FIG. 1 shows a schematic cross section of an embodiment of
the acoustic sensor according to the present invention with the
section plane passing parallel to, and through, four ducts. A
cavity 2 is defined by end walls 3 and 4, and a side wall 5. A
backer, or stiffener plate 11 (which two terms are used
interchangeably herein), attaches to a disc 6. An aperture 13 in
the disc 6 and a duct 12 in the backer 11 allows for fluid
communication between the cavity 2 and the surroundings 14. The
surroundings here defined as being regions both above and around
the sensor. The cavity 2 and a cylindrical hole 1 are substantially
circular in cross section as drawn, similar to the pump disclosed
in WO2006/111755 and the acoustic sensor disclosed in WO2013/083978
which also contain substantially circular cavities. While this
example in FIG. 1 has a cavity with a circular cross-section, it
will be apparent to one skilled in the art that other shapes can be
used. Substantially circular cross-sectional shapes provide
advantages in favourable matching between the displacement of the
end wall and the mode shape of the fluid in the cavity at resonance
as discussed in WO2013/083978. Similar shapes including, but not
limited to, a cavity with a cross-section in the shape of an
ellipse, hypoellipse or hyperellipse share many of these benefits.
In operation the sensor generates radial acoustic pressure
oscillations. In particular, when the cavity radius a is greater
than 1.2 times the height h of the cavity, i.e. a/h is greater than
1.2, the lowest frequency radial mode of oscillation of the fluid
in the cavity has a lower frequency than any longitudinal modes of
the cavity. In operation it is preferable that the lowest frequency
radial mode is at a frequency of 20 kHz or higher so that the
device is inaudible in operation. A frequency of approximately 20
kHz or higher provides operation above the threshold of normal
human hearing.
[0057] The duct arrangement described above allows the apertures in
the end wall to be fixed at nodal points of the radial acoustic
mode, if preferred, while the effective diameter of the transducer
is reduced. As only a small amount of material is removed to form
these small, narrow ducts, the stiffness of the structure is not
impacted significantly; allowing the mechanical resonant
frequencies of the transducer to be raised and high speed of sound
gases to be accommodated by the sensor. Although the cross
sectional area of these channels is small, diffusion times are not
significantly worsened relative to an open design because the
length of these channels is small in comparison to the height of
the backer and cavity diameter.
[0058] To avoid inefficient operation resultant from high viscous
losses in the fluid in the cavity, the height of the cavity should
be at least twice the thickness of the viscous boundary layer in
the fluid:
h > 2 2 .mu. a .rho. k 0 c ; k 0 .apprxeq. 3.83 . [ Eqn . 1 ]
##EQU00001##
[0059] Where .mu. is the viscosity of the fluid, .rho. is the
density of the fluid, c is the speed of sound in the fluid and
k.sub.0 is the first root of the derivative of the first order
Bessel function of the first kind which describes the shape of the
lowest frequency radial mode pressure oscillation and is constant.
Rearranging the above expression and substituting in standard
values for density and viscosity;
h 2 a ##EQU00002##
should be greater than 1.times.10.sup.-7 m when the fluid in the
cavity is a gas and greater than 4.times.10.sup.-10 m when the
fluid in the cavity is a liquid. The height of the cavity of such a
sensor is typically less than 8 mm, leading to a compact device. In
addition to the obvious commercial benefits of a compact sensor,
this also reduces the fluid volume in the device increasing the
speed at which the sensor can respond to changes in fluid
properties.
[0060] In the embodiment shown in FIG. 1, the first end wall 3 is
defined by the lower surface of the disc 6. The sidewall 5 is
defined by a ring 7. The transmitter comprises a piezoelectric disc
8 attached to the disc 6. When an appropriate electrical drive is
applied the transmitter is caused to vibrate in a direction
substantially perpendicular to the plane of the cavity 2, thereby
generating radial pressure oscillations within the fluid in the
cavity. The motion of the transmitter is constrained along a
perimeter defined by the cylindrical hole 1 in the backer 11. The
backer is comparatively thicker and stiffer than the disc, which
means that the inner region of the disc which interfaces with the
cylindrical hole moves substantially more in operation than the
outer region of the disc that interfaces with the backer. The
piezoelectric disc 8 is not required to be formed of a
piezoelectric material, but may be formed of any electrically or
magnetically active material such as, for example, an
electrostrictive or magnetostrictive material. As such, the term
"piezoelectric disc" is intended to cover electrostrictive or
magnetostrictive discs as well. A duct 12 extends from the
cylindrical hole 1 and aligns with an aperture 13 in the disc 6
which forms a fluidic path between the cavity and the surroundings.
At least one of these fluidic paths allows fluid to enter the
cavity (inlet) and at least one of these apertures allows fluid to
leave the cavity (outlet). The fluid may be transported by
diffusion, in which case it may enter and leave the cavity by the
same aperture.
[0061] FIG. 1B shows a typical displacement profile of the first
end wall of the cavity along a line bisecting the cavity. The solid
curved line and arrows indicate the wall displacement at one point
in time and the dashed curved line its displacement one half-cycle
later. Note that the displacements as drawn in this figure and the
other figures are exaggerated and the piezoelectric discs are
omitted from the drawing for clarity.
[0062] FIG. 1C shows a typical pressure oscillation profile for the
cavity shown in FIG. 1A and FIG. 1B. The solid curved line and
arrows indicate the pressure at one point in time, and the dashed
curved line the pressure one half-cycle later. The radial
dependence of this pressure oscillation p(r) approximates the form
of the Bessel function:
p ( r ) = p 0 J 0 ( k 0 r a ) ; k 0 .apprxeq. 3.83 . [ Eqn . 2 ]
##EQU00003##
[0063] Where p.sub.0 is the maximum amplitude of the pressure
oscillation, J.sub.0 is the first order Bessel function of the
first kind, k.sub.0 is the first root of the derivative of that
function, r is the radial position in the cavity and a is the
radius of the cavity. In this case, the motion of the first end
wall 3 and the pressure oscillation in the cavity show significant
mode-shape matching. The embodiment disclosed in FIG. 1 shows an
acoustic sensor wherein, in use, the lowest frequency, also known
as lowest order or fundamental, mode of radial pressure oscillation
is generated in the cavity. An acoustic sensor can be embodied
wherein, in use, pressure oscillations with a radial mode
possessing a resonant frequency higher than the fundamental mode
are generated. These higher frequency oscillations are referred to
as higher order radial modes of pressure oscillation in the
cavity.
[0064] The pressure oscillation in the cavity shown in FIG. 1C
drives a displacement in the second end wall 4. This displacement
is shown in FIG. 1D. No phase relationship between the oscillations
shown in FIGS. 1B, 1C and 1D should be inferred.
[0065] The receiver comprises a piezoelectric disc 9 attached to a
disc 10. The displacement of the second end wall 4 causes an
electrical signal to be generated by the piezoelectric disc 9. By
comparison of the signal used to drive the transmitter and the
signal generated by the receiver, properties of the fluid in the
acoustic cavity can be determined such as the composition of a
binary mixture of gases, temperature of a known fluid, density of a
fluid or viscosity a fluid.
[0066] The acoustic sensor disclosed here can be used to measure
the speed of sound in a fluid or a mixture of fluids. The resonant
frequency of pressure oscillation f.sub.0 disclosed in Equation 3
is described by the expression:
f 0 = k 0 c 2 .pi. a ; k 0 .apprxeq. 3.83 . [ Eqn . 3 ]
##EQU00004##
[0067] Where k.sub.0 is the first root of the derivative of the
first order Bessel function of the first kind, c is the speed of
sound in the cavity and a is the radius of the cavity. The resonant
frequency of the pressure oscillation can be determined by
observing the frequency at which the electrical amplitude gain
between the transmitter and the receiver is maximised. One way to
embody this would be to sweep the frequency of the electrical
driving signal to the transmitter, recording the gain at each
frequency. Similarly, the resonant frequency can be determined by
comparing the phase of the input signal of the transmitter to the
output signal of the receiver. One way to embody this would be to
sweep the frequency of the drive signal and record the frequency at
which the phase difference between the transmitter and the receiver
is some constant value. FIG. 2A shows both the phase difference
between transmitter and receiver and the electrical gain in the
vicinity of the operational cavity resonance for an embodiment of
the sensor described herein. One application enabled by accurate
measurement of the speed of sound of a fluid is the determination
of the composition of a binary mixture of gases. The equations to
relate the composition of a binary gas to the speed of sound of a
fluid are well known.
[0068] FIG. 2B shows the typical gain between transmitter and
receiver over a frequency range which includes the cavity resonance
and lowest mechanical resonant modes of the sensor. In this
instance the acoustic resonance is positioned at .about.23 kHz and
the lowest mechanical resonance is positioned at .about.38 kHz. In
frequency space, for a given gas, the further the lowest mechanical
mode of the sensor is from the cavity resonance, the larger the
range of speeds of sound that can be monitored.
[0069] FIG. 3A shows the bottom view of a backer with several ducts
1-4 that interface with the cylindrical hole 5. The face of the
backer shown interfaces with the disc that forms one of the end
walls of the cavity of the sensor. Four ducts are depicted although
the number may be more or less than this depending on the
performance requirements of the sensor. For example, more ducts may
be preferred if a shorter response time is desired. Conversely, if
the response time of the sensor is not a key consideration, the
number of ducts may be reduced to ensure the mechanical stiffness
of the backer-end wall sub-assembly is as high as possible.
Consequently, this allows as wide a range of speeds of sounds as
possible to be measured.
[0070] The ducts depicted in FIG. 1 and in FIG. 3A interface with
the cylindrical hole but different arrangements are possible. FIG.
3B shows the bottom view of a backer with ducts 1-4 that do not
interact with the cylindrical hole 5 but instead extend radially
outwards to the surroundings 14.
[0071] FIG. 3C shows a cross section of an embodiment of the
acoustic sensor that incorporates the backer design depicted in
FIG. 3B where a duct 12 aligns with an aperture 13 in a disc 6. The
duct 12 extends radially outwards to the surroundings 14 to
minimise the fluidic path length between the cavity and
surroundings. A short fluidic path length is desirable in many
applications where diffusion of the fluid into the cavity limits
the response time of the acoustic sensor to changes in the
composition of the fluid in the cavity.
[0072] FIG. 4 shows the bottom view of an embodiment of an end
wall. In this embodiment the end wall has four apertures 1-4. In
order to maximize the quality factor of the acoustic resonance
excited in the cavity, the apertures should be positioned at nodal
pressure positions of the resonant acoustic mode so that acoustic
damping originating from oscillatory fluid motion in and out these
apertures is minimized. In this embodiment the apertures are
positioned at nodal positions of the first order radial acoustic
mode but these may also be positioned at the nodal positions of
other higher order modes.
[0073] FIG. 1 shows an embodiment of the acoustic sensor that is
symmetric about the plane which bisects the cavity at its
mid-height. However, in some applications it may be desirable for
the geometry of the acoustic sensor to be non-symmetric about this
plane. For instance, it may be desirable to have fluid entry into
the cavity and fluid exit from the cavity from one side of the
sensor only. FIG. 5 shows an embodiment of the acoustic sensor
where an end wall 3 has an aperture 13 which aligns with a duct 12.
The duct 12 interfaces with a cylindrical hole 1 which interfaces
with the surroundings 14. A second end wall 4 has no apertures and
is solid throughout. Consequently, the second end wall 4 does not
permit fluid to pass through it and fluid communication between the
cavity and the surroundings is achieved through apertures in the
first of the end walls only.
[0074] While it may be preferable for respective backers, or
stiffener plates, to be attached to each of the respective end
walls, embodiments are envisaged where only one backer is attached
to only one end wall.
[0075] The (each) end wall and the (each) backer/stiffener plate
are preferably distinct articles, the backer/stiffener plate being
attached to the end wall, by some suitable means (e.g. bonding,
welding, brazing, fixing) in the assembled sensor. Alternatively,
the (each) end wall and the (each) backer/stiffener plate may be of
unitary construction. That is, in an embodiment the end wall and
backer/stiffener plate are made integral with one another. In such
an embodiment, the backer/stiffener plate may be considered to be a
stiffener part, or stiffener portion, of the sensor.
[0076] It will be understood that the invention has been described
in relation to its preferred embodiments and may be modified in
many different ways without departing from the scope of the
invention as defined by the accompanying claims.
* * * * *